Nanoparticles are generally defined as particles having a diameter of less than 1 micron. Nanoparticles may be used in a variety of processes within the oil-and-gas industry, such as enhanced oil recovery, clay stabilization, drilling fluids, fracturing fluids, etc. Nanoparticles may also be used for products such as dyes and pigments, coatings, magnetic recording media, quantum dots, and semiconductors.
Nanoparticles may be used to form suspensions or colloids. There are certain challenges in forming and maintaining suspensions of nanoparticles. For example, particles tend to aggregate more quickly in liquids having higher ionic strengths, and aggregates tend to settle from the suspension.
Solid surfaces exposed to a liquid exhibit a structure referred to in the art as an electrical double layer, in which two layers of oppositely charged particles (e.g., ions and electrons) cover the surface. In the case of nanoparticles, the layers may surround the nanoparticles. As the size of the nanoparticles decreases, the effect of the charged layers on the nanoparticles' stability in the liquid increases. Furthermore, the size of the electrical double layer decreases as the ionic strength of the liquid increases. A decrease in the electrical double layer corresponds to a decrease in repulsive forces between nanoparticles. Because repulsive forces generally limit the rate of aggregation of nanoparticles, large ionic strengths of the liquid therefore allow attractive van der Waals forces between nanoparticles to dominate their movement, and dispersions of nanoparticles in high-ionic-strength liquids tend to become unstable and form sediment (i.e., larger agglomerations of particles that fall from the suspension). With an increase in temperature, the kinetic energy of suspensions increases, which leads to more frequent and higher-energy particle collisions, which further disrupt the electrical double layer and cause nanoparticles to aggregate.
Colloidal nanoparticle suspension may generally be controlled through charge stabilization, steric stabilization, or a combination of both charge and steric stabilization. Colloidal stability of nanoparticles is important for certain applications in aqueous media containing high electrolytes like biological fluids, sea water, high-brine concentrated injection fluids, etc. Many practical applications in biomedical, environmental, and oil and gas applications use high-ionic-strength liquids. In such applications, nanoparticle stability is crucial.